1. Field of the Invention
The present invention relates to a method and a device for determining the flow rate of a fluid flowing in a channel, the fluid being constituted by a liquid or a gas.
2. Description of Related Art
It is previously known to determine the flow rate of a fluid flowing in a channel by sending two pulse shaped, oscillating signals through the fluid, one of the signals being directed against and the other signal being directed with the flow of the fluid, whereupon the signals are received and the flow rate is determined by means of the phase shift between the received, pulse shaped signals, caused by the fluid flow. The signal sent with the fluid flow will more rapidly reach the signal receiver than the signal sent against the fluid flow, and this provides for a phase shift between the two signals by means of which it is possible to determine the flow rate. It is when using this method for determining the flow rate known to use a device consisting of two signal generators positioned at a distance from each other in the direction of the fluid flow, the signals being sent from each of the signal generators against the other signal generator, the signal generators being switched over to signal receivers before the transmitted pulse shaped signals arrive to the signal generators. As the speeds of the signals in the fluid are different at different fluids and also in one and the same fluid if this has different pressures and temperatures the dwell time of the pulse shaped oscillating signals in the fluid will be different which causes that the phase shift between the signals is different even if the flow rates of the fluids are the same. This is of course a great disadvantage of the previously known method for determining the fluid rate as the determination is thereby less exact.
In another previously known method for determining the flow rate of a fluid flowing in a channel there is sent a pulse shaped, oscillating signal from a first point to a second point which points are positioned at a distance from each other in the direction of flow, whereupon the signal arriving to the second point is digitized and the digitized representation of the signal is registered in an electronic memory. There upon a pulse shaped, oscillating signal is sent in the opposite direction, i.e. from the second to the first point, whereupon the signal is in the same way digitized and registered in the electronic memory. Because of the fact that one signal moves with the flow and the other signal moves against the flow in the fluid there is provided a phase shift between the signals, and this phase shift is used for determining the flow rate.
In this previously known method it is a drawback that the signals are not sent and received at the same time which provides that a temporary variation in the flow rate will have an influence on the determination of the flow rate. A further drawback is that extremely expensive circuits are required for conducting this method.
The object of the invention is to provide a method and a device for determining the flow rate of a fluid flowing in a channel, which obviates the lacking exactness in the methods described above and makes it possible to use less expensive circuits for conducting the different functions.
In order to comply with this object the method according to the invention is characterized in that the determination of the phase shift between the signals is conducted while compensating for the dwell time of the pulse shaped oscillating signals in the fluid. Thus, the phase shift between the signals is corrected with regard to the speed of the signals in the fluid so that the phase shift which is utilized for the determination of the fluid rate is constituted by the phase shift between the signals present at one and the same dwell time for the signals in the fluid.
It is in the method according to the invention preferred that the two pulse shaped oscillating signals are concurrently sent from one signal generator each, the signal generators being positioned at a distance from each other in the direction of flow of the fluid, towards one signal receiver each, wherein it is suitable to use signal generators which can be switched over to signal receivers. Thereby, the two pulse shaped oscillating signals are sent from each of the signal generators towards the other signal generator and the signal generators are switched over to signal receivers before the transmitted pulse shaped signals arrive thereto.
In the method according to the invention it is preferred that the signals are sent and received at different sides of the channel and that the determination of the phase shift between the signals is conducted under compensation also for the angle between the direction of movement of the signals and the direction of flow of the fluid.
It is suitable that each of the signal generators is supplied with a sinusoidal, pulse shaped voltage which by means of the signal generators is converted into an ultrasonic signal which is through the flowing fluid supplied to the signal receiver and is thereby converted into a sinusoidal pulse shaped voltage. The sinusoidal, pulse shaped voltages from the signal receivers are supplied to an analogue phase detector for determining the phase shift between the sinusoidal pulse shaped voltages. Thereby the channel for the flowing fluid can be constituted by a pipe which consists of a material allowing the ultrasonics to pass without too much damping or abduction. However, the pipe walls should consist of a relatively homogenous material which provides for better and cleaner signals. I order to avoid the problems which can appear when the ultrasonic passes the pipe walls it is possible also to position the signal generators inside the pipe.
The invention comprises also a device for conducting the method.